Modern communication systems use mixers as primary frequency-translation devices. Mixers can be either analog-multiplying-type or switching-type, and both types have found extensive use in communication receivers and transmitters. Analog multiplying mixers generally perform the straight multiplication of the inputs without generating additional components at the output. By contrast, switching mixers generate a multiplicity of components at the output typically due to the hard switching of one of the (linear) inputs by the second input; usually, only one of these components is desired and the rest are either filtered out or eliminated by other means.
In general, switching mixers are more popular than analog multipliers in integrated circuits due to their simpler design, lower noise, better dynamic range, and higher linearity. In the case of wideband systems such as receivers for digital television signals, eliminating the undesired components caused by the harmonics of the commutating local oscillator (LO) signal of the mixer is difficult to achieve by filtering only, especially at the lower end of the frequency range. For the efficient elimination of the harmonic products in these systems, harmonic rejection mixers are frequently used.
A harmonic rejection mixer typically consists of a multiphase local oscillator generator and a multiplicity of individual switching mixers followed by a scaling circuit and an output adder, as shown in the block-diagram representation of FIG. 2 where the phase accuracy of the multiphase LO generator signals is critical for the efficient suppression of the harmonic mixing products.